theoretically, you have to drill out all the spot welds - I found it easier just to cut out the panel on the insides of the overlap, and then grind down the welded flaps after.

It took me 2-3hrs to cut it all out (that's after the bumper & trunk interior was removed), and another 3 hours to grind out the flaps and weld the new one in. I just tack welded it. Then, to do a clean job, you need to reapply seam sealer.

If you have rust forming my advice - TREAT IT RIGHT NOW

It's a pain to take the whole part out. You're better off cleaning it up asap, or just cutting out & having a custom piece installed in the local area.